The Supreme Court of Cambodia has dissolved the Cambodia National Rescue Party (CNRP), the most popular opposition party in the country.
The ruling caps an apparent crackdown on dissenting voices launched by the prime minister ahead of next year's parliamentary elections.
He said 118 opposition party members would also be banned from politics for the next five years, and the verdict could not be appealed.
The CNRP dissolution and political bans came after the government arrested CNRP's leader Kem Sokha in early September for plotting the overthrow of the government.
"It shows that Hun Sen will never stop if no one is stopping him", said Ms Kem Monovithya, the daughter of Mr Kem Sokha and also a party official.
Hun Sen has encouraged opposition lawmakers to defect to his ruling party before Thursday's ruling. "The verdict is expected". He and his ruling government were almost defeated in the last national election in 2013, and support is growing for the opposition, especially among younger Cambodians eager for change.
Prime Minister Hun Sen has been leading the CPP in Cambodia for almost 33 years, and many experts see his government's dismantling of the opposition CNRP - a party that almost dethroned the ruling party in the 2013 general election - as an attempt to corner the competition and guarantee the CPP the July 2018 national election.More news: Yankees' Luis Severino finishes third in AL Cy Young race
At the same time, Cambodia has in recent years moved closer and closer to China, its top aid benefactor and investor, and a country that does not preach the virtues of democracy and human rights. The United States and European Union missions in Cambodia declined immediate comment on the court ruling.
"The Supreme Court's decision today is not to end democracy but to deter extremists in order to protect the people and the nation from destruction", said Huy Vannak, undersecretary of state at the interior ministry.
As the election gets closer, Hun Sen has been steadily chipping away at the strength of the Cambodia National Rescue Party, the country's only opposition party, whose top leaders are either behind bars at home or in self-exile overseas.
Chief Judge Dith Munty, who is a senior ruling party member, announced the nine-member court's unanimous ruling.
Sam Rainsy said on Wednesday in a Facebook post that he was returning to the party, and said the CNRP would remain in the hearts of Cambodians even it was dissolved.
The lawyers screened videos of Sokha that documented him admitting to having received assistance from the USA in planning his political career and also played audio recordings from Radio Free Asia that they said proved there was a link between the U.S. and the opposition in planning a colour revolution.
Rainsy used to lead the Cambodia National Rescue Party but went into exile a year ago because of the threat of prison in legal cases against him that are widely regarded as politically motivated.